The present invention relate s to cont rollers for charging generators, and particularly to a control device for a charging generator having a protection circuit suitable for the control device that is installed in vehicles and has a radio noise reducing circuit such as a soft switching circuit for reducing radio noise, and for protecting a electric power element for controlling a field current from being broken down when the field coil of the generator is short-circuited by some cause so that a battery voltage is supplied directly to the electric power element.
There is known a short protection device that is designed to protect the field current control electric power element from being broken down when the field coil is short-circuited as disclosed in, for example, JP-A-6-276695.
The protection device of this kind has a short protection circuit which decides that the field coil is short-circuited when the voltage at the junction between the field coil and the electric power element is equal to the battery voltage, making the electric power element nonconductive. In this case, a PWM (Pulse Width Modulation) output for controlling the electric power element has a fixed frequency and a variable duty, so that the electric power element can be switched from the conductive state to the nonconductive state in a short constant period of time. Thus, even when the electric power element is in the short-circuited state during that short period of time, it can be made conductive by the PWM output, and the short-circuit detection circuit can be reset.